The present invention relates to the field of opto-electronic directional couplers.
Prior art directional couplers are interferometric two-by-two light beam switches where both channels are preserved for both the bar and cross states. The waveguide channels are optically coupled at centralized portions thereof.
The nature of delta-beta and delta K directional couplers are also well known. See example U.S. Pat. No. 4,756,588, issued to Granestrand and the references and publications cited therein.
In U.S. Pat. No. 4,778,235, the inventor, Fujiwara discloses switches which utilize the lateral spreading or fanning out of light, to produce the desired light losses at selected legs of the switch. See FIG. 6 in particular, and col. 7. His switches are "gain guided" so that at zero bias, portions of his waveguide "disappears". In sharp contrast, the switches of the present invention employ index-guided-channel waveguides, having strip-loaded ribs, buried inverted ribs, or other embedded corrugations, producing a lateral index step for horizontal confinement of light as well as the vertical confinement. In the present invention, band-to-band optical absorption in the waveguide at zero or reverse bias, produces the desired optical attenuation, in contrast with the lateral fanning-out of light which occurs in waveguide portions of all of Fujiwara's devices. As a result, our on-off light contrast ratio will be 1000/1 or more in contrast with Fujiwara result, estimated at 100/1 or less which will thus produce more crosstalk than switches of the present invention. Regarding his "directional coupler" of FIGS. 3 and 4, one of the waveguides will disappear so that, in contrast with the present invention, he doesn't teach a true 2.times.2 directional coupler switch where both data channels are preserved at both outputs, for both the bar and cross states, and thus light-modulated data would be lost.
Although he suggests utilizing refractive index changes and gain differences in what he calls a directional coupler, he cannot preserve both data channels, as one waveguide disappears. That is, he never has two index-guided channels for a true 2.times.2 interferometric directional coupler.
U.S. Pat. No. 4,737,003 to Matsumura teaches quantum well switching devices with carrier injection at cross-over guide portions. These however are of the deflection or refraction type.
Directional coupler switches of the delta-beta type are also described in U.S. Pat. No. 4,784,451 to Nakamura et al; see col 6, lines 38, through the remaining detailed description. However, in contrast with the present invention, these devices are reversed biased and employ the pockels effect to change the index of refraction. The result is that strong electric fields and relatively few carriers are induced into the semiconductor material which limits the change in the index of refraction. U.S. Pat. No. 4,720,174 to Baker discloses a biased liquid crystal for switching a directional coupler. Our switching speeds are much faster and employ all solid state waveguides.